Y-Cable Summing

Hello,

I have a stereo LA2a that I'd like to sum the outputs of into a Tube Tech CL1B. I can use the gain knobs on the LA2a to mix the two signals. I'm thinking a simple Y-cable XLR will be fine.

I built a summing Y-cable before and if I remember correctly it was just a matter of putting a resistor on the hot and cold pins coming out of the stereo output, nothing on the ground, then just tie the wires on the mono XLR input..

What I'm wondering is what resistor value to use? The Tube Tech has a 600 ohm input.

Thanks,
Bobby

The classic formula for passive mixing networks is

R = (N-1) / (N+1) * Z

Where N = number of sources to be mixed
and Z = desired input impedance

If you want the output impedance of the mixing network to be the same as the input impedance, you add another resistor (again equal to R) in series with the output.

If you're doing it balanced, each resistor is R/2

http://electronicdave.myhosting.net/miscimages/mixnetwork.gif

Dave,

Thanks for the info and pic.

So for my application:

R = (N-1) / (N+1) * Z

R = (2-1) / (2+1) x 600
R = 200
R/2 = 100

Is this correct for 2x1 balanced inputs into 600 ohms?

That gives me a value of 100 ohms on each resistor. I also see I should be putting a resistor on ground too? The value seems low to me but I suppose that's because the Tube Tech CL1B has an unusually low input impedence, correct?

Thanks again,
Bobby

100 ohms on the inputs to the summing network (big name for "y-cable") works fine.

Thanks Dave.